Environmental Engineering Reference
In-Depth Information
an anthropogenically-enhanced nutrient supply from land or where upwelling
brings up subsurface waters of low O
2
content that is further reduced through
degradation of copious, locally-produced organic matter. In both cases, a high
organic loading is the primary cause of O
2
depletion. This is often associated
with strong near-surface stratification, which inhibits vertical mixing and as-
sociated aeration of subsurface waters. There are several examples of the first
type of O
2
deficient environment, popularly known as 'dead zones' because
of exclusion of many organisms including commercially important fishes. The
largest and the best investigated 'dead zone' along an open-ocean coast devel-
ops every summer in the inner Gulf of Mexico as a consequence of fertilizer
runoff by the Mississippi [43]. Coastal O
2
depleted environments of the second
category are primarily of natural origin, found along the eastern boundaries
of the Pacific and the Atlantic Oceans and along the northern boundary of the
Indian Ocean [25, 28]. The total areas of the continental margin containing
waters with O
2
<
0.5 mL L
−
1
(22 µM) and
<
0.2 mL L
−
1
(9 µM) are estimated
to be 1.15 x 10
6
and 0.76 x 10
6
km
2
, respectively. The majority of these areas
(59 and 63%, respectively) are found in the Indian Ocean north of the equator
[25]. This is because the North Indian Ocean is semi-enclosed, bounded by
land at low latitudes. As a consequence, an overall high biological production
combines with a slow re-oxygenation at depth to produce some of the most
intense O
2
depletion observed anywhere in the open ocean [35, 54, 57] within
a depth range that includes a large portion of the continental margin.
Pelagic O
2
deficiency strongly influences biodiversity, ecosystem function-
ing and biogeochemical transformations. One of the most important biogeo-
chemical consequences is the activation of alternate (anaerobic) pathways of
respiration, mainly denitrification and sulphate (SO
4
2
−
) reduction. The former
involves the reduction of oxidized nitrogen [mainly nitrate (NO
3
−
)] to molecu-
lar nitrogen (N
2
) when O
2
concentration falls close to zero, and the latter leads
to the production of hydrogen sulphide (H
2
S) when the water is completely
stripped of both O
2
and NO
3
−
[46]. Oxygen depletion in the North Indian
Ocean is severe enough to allow these processes to occur in the water column.
The countries bordering the North Indian Ocean account for approximately
a quarter of the world's human population, a great majority of which (about
1.4 billion people) lives in the three major South Asian countries - India,
Pakistan and Bangladesh. The requirements of food and energy for such a
large population are obviously enormous. For example, in order to sustain
agricultural production, the South Asian countries consumed in 2002-2003
about 14 million tonnes of nitrogen (N) as synthetic fertilizer (out of the global
consumption of about 85 million tonnes); this represents a roughly 46-fold
increase over the amount used in 1960-61 [27]. Similarly, oil consumption,
which accounts for roughly a third of all commercial energy sources in the
